JP-A-2000-066240 (Laid-open Publication of Japanese Patent Application) discloses a halftone exposure technique effective to reduce the required number of steps in a photomasking process in the manufacture of a liquid crystal display device of active matrix type. The halftone exposure technique disclosed in this publication uses a slit-type photomask 70 which has slits 73 formed in its local portions, as shown in FIG. 1. The locally formed slots 73 serve to adjust an average amount of exposure of an active matrix substrate 9 to a light generated by an exposing device, for forming a first pattern of a resist on a portion of the substrate 9 which corresponds to a channel portion of a thin-film transistor. The first pattern of resist has a positive resist portion 6 having a relatively large thickness and a positive resist portion 7 having a smaller thickness than that of the positive resist portion 6. Then, an etching operation is performed on the substrate 9 so that only those portions of semiconductor layers 10, 11, a barrier metal layer 12 and a low-resistance metal layer 13 which underlie the positive resist portions 6, 7 of the first resist pattern are left, whereby the semiconductor layers 10, 11 are formed into semiconductor elements. The thickness of the positive resist portions 6, 7 are then reduced by an ashing operation, so that the positive resist portion 7 in the area corresponding to the channel portion of the thin-film transistor element is removed, with a result of formation of a second resist pattern which consists of only the positive resist portions 6. The channel portion of the thin-film transistor element is formed by etching using the second resist pattern. According to the halftone exposure technique disclosed in JP-A-2000-066240 described above, a single photomasking step permits formation of the semiconductor layers into the semiconductor elements, and formation of the channel portion of the thin-film transistor element. Accordingly, the halftone exposure technique makes it possible to reduce the required number of the photomasks, and considerably lower the cost of manufacture of the active matrix substrate, as compared with the conventional photomasking technique. The halftone exposure technique of JP-A-200-066240 is shown in FIG. 22.
Where the halftone exposure technique as disclosed in JP-A-2000-066240 is used to form the channel portion of the thin-film transistor element, however, the formed channel portion of the thin-film transistor element tends to have a relatively large amount of variation in its dimensional accuracy, leading to instability factors in mass production of a product including the thin-film transistor elements. Further, a variation in the amount of overlapping between a gate electrode and source and drain electrodes causes a display variation in the halftone area, giving rise to a problem of reduction in the yield ratio of the active matrix substrate.